The present invention relates to phototubes and more particularly to photomultiplier tubes.
A phototube is an electron discharge device which is particularly useful for detecting an input signal in the form of radiation, focused to impinge upon an input surface of that device. The input surface of such a device generally includes a photocathode within an evacuated envelope for converting radiation into a stream of electrons. An anode is provided for collecting the electron stream, generated within the evacuated envelope, and for providing an electrical output signal current related to real time magnitudes of the collected electron stream.
In photomultipliers, an electron multiplier is interposed within the electron stream between the photocathode and anode to provide, in ordered sequence, one or more electrode stages of electron multiplication. An electric field between the photocathode and the succeeding electrodes acts as an electron lens whereby the various electrons of the electron stream are accelerated as an electron bundle to impinge upon each of the succeeding electrodes in sequence.
Phototubes have generally been limited to their ability to most effectively convert incident radiation into useful anode output signal current. One reason for this limitation has been inability of one or more electrodes (anode or dynode) to completely collect the entire electron bundle consisting of the electrons emitted from the effective cathode surface(s) or a preceding dynode electrode surface. Theoretically, maximum collection of the electrons, or optimum signal output, occurs whenever all the emitted electrons are accelerated by electron lens system to impinge upon or be collected by an active region of a succeeding electrode. In practice, however, electron lens systems are deficient in that they are incapable of providing such optimum focussing of the entire electron stream. Such deficiencies of electron lens systems are particularly acute for phototubes wherein electrons are emitted from a flat photocathode and/or are emitted from the interior side walls of the tube. Furthermore, the actual surface area of the active collection region of succeeding electrodes is limited by internal tube design considerations, and/or other constraints to a total surface area considerably less than that necessary to collect all useable electrons emitted as a result of the input signal.